1. Field of the Invention
The present invention relates to an optical transmission line suitable for transmission and large capacity transmission of signal light and, more particularly, to hybrid optical fibers.
2. Technical Background
Submarine optical fiber cable systems, i.e. those which travel under water, typically between continents and or along the coastline of continents, can be repeatered or unrepeatered. Repeatered submarine systems employ repeaters along their length. A repeater is a unit typically in the form of an enclosed box, which contains an amplifier to boost signal strength and an equalizer to correct distortion. Typically such repeaters are placed at intervals along the submarine cable to allow longer cables to be used. Conversely, unrepeatered systems are submarine cable systems which do not use repeaters. Unrepeatered systems typically do not extend further than about 500 kilometers, and in most instances are considerably shorter than 500 kilometers. Unrepeatered systems are very often used in festoon applications, wherein the submarine cable is displaced between a transmitter and receiver at different locations along a coastline, with the festoon fiber being deployed under water between the transmitter and receiver.
Unrepeatered systems are playing a valuable role in linking markets, particularly those separated by distances of 100 to 500 km. One type of these systems, classified as festoon, is comprised of undersea networks which are used connect islands (in Malaysia, for example) as well as to circumvent unstable geological or political routes (in Africa and South America, for example). Several unrepeatered systems have also been installed in Northern Europe and across the English Channel, as well as on long overland routes that do not require add/drop capabilities but could use branching units. These applications are viewed as rapidly growing by most market analysts because they offer an affordable method of transmitting optical signals over several hundred kilometers.
The most commonly used fiber in unrepeatered systems is conventional single mode fiber, due its combination of low attenuation, large effective area, low price and wide availability. Such standard singlemode fiber may be comprised of a germania doped silica fiber, such as Corning SMF-28(trademark) fiber, or pure silica core fiber. More recently, large effective area cutoff-shifted fibers such as Corning Vascade(copyright) L1000 have become available, which offer significantly larger effective areas (about 95-105 sq xcexcm at 1550 nm) than standard single mode, and a cabled cutoff wavelength above the 1310 nm window. For comparison, the cutoff and effective area of standard single mode fiber are  less than 1260 nm and about 80 sq. microns, respectively. Attenuation values of 0.194 dB/km and 0.186 dB/km have been reported for standard singlemode and cutoff-shifted fibers, respectively.
Fiber attenuation is a key attribute in the design of an unrepeatered system, as losses between approximately 20 and 60 dB are incurred when span lengths of 100-300 km are employed for a fiber having an attenuation of 0.20 dB/km, for example. Signal to noise constraints generally constrain system lengths to less than about 220 km when the sole source of amplification is an EDFA at the transmitter. The addition of distributed Raman amplification may increase the maximum length by 50 to 100 km. The Raman pump lasers are usually backward-propagating from the receiver end. Distances of 350-500 km can be achieved through the use of one or more Remote Optically Pumped Amplifiers (ROPAs), which consist of a length of Erbium-doped fiber which is spliced into the transmission path and pumped using the same fiber or an additional length of fiber that is optically coupled to the transmission fiber near the Erbium-doped section.
A second key attribute in the design of unrepeatered systems is effective area. The need to increase the channel count per fiber requires higher power handling capacity. The approximately 80 sq. micron effective area of standard single mode fiber offers acceptable performance up to distances of approximately 200 km and channel spacings of 50 GHz (0.8 nm) or higher. Increasing the distance requires higher input powers, which in turn increases nonlinearities such as self-phase-modulation (SPM). Decreasing the channel spacing results in increased penalties from inter-channel effects such as cross-phase modulation (XPM) and four-wave-mixing (FWM) if there is not a concomitant increase in the fiber effective area. It is therefore difficult to increase the product of capacity times distance in systems employing only standard single mode fiber. However, increasing the effective area of standard single mode from about 80 to about 101 sq. microns (typical of Corning Vascade(copyright) L1000) allows a (101-80)/80=25%=1 dB increase in the input power without an increase in fiber nonlinearities.
Standard single mode fibers have inherently high dispersion ( greater than 17 ps/nm/km) which must be compensated for at the terminals. A typical DCM-100(trademark) dispersion compensation module, for example, compensates for up to 1700 ps/nm of dispersion and has a loss of about 10 dB. Thus, a 250 km link, which requires about 4,250 ps/nm of dispersion compensation at 1550 nm, will require at least two such modules placed between amplifiers to overcome the loss. This adds considerably to the complexity and cost of the terminal equipment. It is therefore desirable to reduce the average dispersion of the transmission link.
It is also desirable to maintain low attenuation in the transmission line, as the deterioration in the Optical Signal to Noise Ratio (OSNR) associated with increased attenuation is the dominant signal degradation mechanism in unrepeatered transmission systems.
Optical fibers of different physical characteristics are combined to produce a hybrid fiber which, at the operating wavelength, has a relatively large effective area and desirable net dispersion and attenuation characteristics over the combined length of the hybrid system. Hybrid fibers are constructed in accordance with the principles of the invention by optically connecting a first fiber having a large effective area (greater than 85 xcexcm2), an attenuation less than 0.19 dB/km and a dispersion greater than 10 ps/nm/km at 1550 nm, to a second fiber having an effective area which is smaller than the first fiber, a dispersion which is less than 10 ps/nm/km, and an attenuation which is less than 0.23 dB/km at 1550 nm. Both the first and second fibers preferably have a positive dispersion slope at 1550 nm. The first and second fibers are preferably spliced directly together.
The hybrid optical fiber may be employed in a telecommunications system. In a preferred embodiment of the invention, the segment of the hybrid fiber that has the largest effective area is positioned nearer to the transmitter end of the system. Placement of the segment of fiber with the largest effective area at the transmitter end of the hybrid fiber span ensures that the optical power density is reduced where the signal is most intense because the optical mode of the signal is spread over a larger effective area. This, in turn, minimizes the undesirable nonlinear effects in the fiber. The lengths of the first and second fibers are selected so that the average attenuation of the span is less than 0.2 dB/km at 1550 nm. The lengths and dispersion characteristics of the first and second fibers are also selected preferably to result in an average dispersion, over the length of the fiber span, which is less than 10 ps/nm/km at 1550 nm.
If the total dispersion of the transmission line can be reduced, then the equipment associated with dispersion compensation (e.g. DC modules) and associated amplifiers can also be reduced. The low dispersion hybrid fiber solutions disclosed herein offer significant reduction in terminal component count and hence system cost, which has been one of the major advantages of the use of NZDSF fibers in terrestrial networks.
Additional features and advantages of various embodiments of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention.